What I meant with the Digg comparison is an "implosion" scenario whereby Facebook does something disastrous to itself which causes a mass exodus, as opposed to

What I meant with the Digg comparison is an "implosion" scenario whereby Facebook does something disastrous to itself which causes a mass exodus, as opposed to something better coming along to supplant it. I think it would be very difficult for it to happen any other way. Indeed, the points you make reinforce this idea.